The present invention relates to an apparatus for measuring characteristics of electronic devices, such as transistors, etc.
An electronic device measurement apparatus, generally known as a curve tracer, is useful for measuring characteristics of basic electronic devices, such as transistors, etc. The construction of a conventional curve tracer is shown in FIG. 4. FIG. 4 shows a floating collector supply circuit 10, which generates a collector voltage, for example by full-wave rectifying an AC voltage from an AC line voltage source, because it is necessary to apply only one polarity (positive or negative) of the voltage to a device under test (DUT) so that the DUT will be protected from destruction by a reverse bias voltage. The rectified voltage from the collector voltage supply circuit 10 is applied through a limiter resistor 16 to the collector of a transistor 18 as the DUT. The value of the resistor 16 is adjusted in accordance with a measurement range. A reference terminal of the collector voltage supply circuit 10 is connected through a current detection resistor 20 to the emitter of the transistor 18 under test as well as being grounded. The base of the transistor 18 receives a bias signal from a bias supply circuit 10, the bias signal being changed step by step. In FIG. 4, the transistor 18 under test is connected as a common emitter type to the curve tracer, however, a common base type or a common collector type may be available. A first voltage detection circuit 24 having a high input impedance detects the voltage V(CE) between the collector and the emitter of the transistor 18 under test and applies the detected voltage to the horizontal deflection plate of a cathode ray tube (CRT) 28 as a display device. A second voltage detection circuit 30 having a high input impedance detects the voltage across the resistor 20, i.e. a voltage corresponding to the collector current I(C) of the transistor 18 under test, and applies the detected voltage to the vertical deflection plate of the CRT 28. Thus, the CRT 28 can display a V(CE)-I(C) characteristic of the transistor 18.
In the electronic device measurement apparatus shown in FIG. 4, one terminal of the DUT, i.e. the emitter of the transistor 18, is grounded, so that the current from the bias supply circuit 22 is prevented from flowing through the current detection resistor 20. In other words, since the bias supply circuit 22 is grounded, the current from the bias supply circuit 22 comes back to the bias supply circuit 22 via the base-emitter junction of the transistor 18 and ground. Thus, a closed circuit is formed by the collector voltage supply circuit 10, the resistor 16, the collector-emitter path of the transistor 18 and the resistor 20, and thereby only the collector current of the transistor 18 will flow through the resistor 20. In addition, the circuit of FIG. 4 has the advantage that the voltage detection circuits 24 and 30 do not need to be floated, since the emitter of the transistor 18 is grounded. It should be noted that if the current detection resistor 20 is inserted in the collector side of the transistor 18, the voltage detection circuit 30 must be floated. In this instance, the circuit may be complex in construction.
There is stray capacitance in an electronic circuit. Similarly, there is stray capacitance as shown by dotted capacitors 15 and 17 in the electronic device measurement apparatus of FIG. 4. These stray capacitors 15 and 17 are representatively shown, and in fact the stray capacitance is distributed throughout the entire measurement apparatus. Thus, a part of the output current from the collector voltage supply circuit 10 flows to ground through the stray capacitors 15 and 17, and the current returns to the collector voltage supply circuit 10 from ground via the current detection resistor 20. Therefore, the current flowing through the resistor 20 comprises not only the collector current of the transistor 18 but also the currents flowing through the stray capacitors 15 and 17, so that an error occurs when detecting the collector current from the voltage across the resistor 20. It should be noted that the output voltage waveform from the collector voltage supply circuit 10 is the full-wave rectified waveform of a sine-wave (AC line voltage waveform) in the conventional electronic device measurement apparatus shown in FIG. 4. Since the full-wave rectified waveform changes rapidly at ground level, the error may be the maximum at ground level and levels near it. The collector current detection error based on the stray capacitance is called a looping error.
Conventional techniques to reduce this looping error are proposed in the 576 curve tracer manufactured by Tektronix, Inc., Beaverton, Oreg. One of the conventional techniques is a guard technique which encloses the collector voltage supply circuit 10 and the resistor 16 with a conductor and applies the voltage at the left terminal of the resistor 20 to the conductor. However, the looping error cannot be eliminated completely by the guard technique alone. Thus, the 576 curve tracer also uses a looping compensation circuit as shown in FIG. 5 for further reducing the looping error. In FIG. 5, the looping compensation circuit comprises variable capacitors 31 and 33 for the stray capacitors 15 and 17 and also a resistor 35. The resistor 35 is inserted between the resistor 20 and the voltage detection circuit 30. Each of the capacitors 31 and 33 has one terminal connected to the right terminal of the resistor 35. The other terminals of the capacitors 31 and 33 are connected to the left and right terminals respectively of the resistor 16. The value of the resistor 35 is the same as that of the resistor 20. The capacitors 31 and 33 are adjusted such that the currents flowing through the capacitors 31 and 33 become equal to the currents flowing through the stray capacitors 15 and 17, respectively. The currents flowing from the collector voltage supply circuit 10 through the capacitors 31 and 33 return to the collector voltage supply circuit 10 via the resistor 35. (It should be noted that the input impedance of the voltage detection circuit 30 is very high.) Thus, the voltage across the resistor 35 produced by the current flowing through the capacitors 31 and 33 is the same as the voltage across the resistor 20 produced by the currents flowing through the stray capacitors 15 and 17. Since the polarities of these voltages are opposite with respect to the input terminal of the voltage detection circuit 30, these voltages are cancelled. Therefore, the circuit of FIG. 5 can compensate the looping error based on the current flowing through the resistor 20 from the stray capacitors 15 and 17.
As mentioned above, the stray capacitance is distributed throughout the entire electronic device measurement apparatus, and the phase of the current flowing through each portion of the stray capacitance is different from each other. Thus, a large number of variable capacitors, similar to the capacitors 31 and 33 shown in FIG. 5, must be provided in a looping compensation circuit. All the capacitors must be variable, because the stray capacitance is different in every electronic device measurement apparatus even if all the electronic device measurement apparatuses are the same in construction. It is necessary to adjust a large number of the variable capacitors to compensate the looping error, and the adjustment is very complex.
The output voltage from the collector voltage supply circuit 10 is the full-wave rectified waveform (FIG. 6B) of a sine-wave, so that this waveform changes rapidly and includes high frequency components around ground level. Thus, the error current based on ground and near ground levels of the full-wave rectified waveform is much larger than the error current at other levels of the full-wave rectified waveform. Because the looping error compensation circuit employing variable capacitors cannot completely compensate the looping error current, the error at each point on the characteristic curves (displayed on the CRT 28) is different from the error at each other point. The measurement accuracy is determined by the worst error, and the overall measurement accuracy may therefore be reduced.
It is, therefore, one object of the present invention to provide an improved electronic device measurement apparatus which improves looping error compensation by a large margin.
It is another object of the present invention to provide an improved electronic device measurement apparatus which can compensate looping error by an easy adjustment.